1. Field of the Invention
This invention relates to a circuit for use in a tone receiver for converting the mutually exclusive outputs of its tone detector device to a multiple digit binary signal representing the presence of a particular tone combination, and for providing output buffering. More particularly it relates to such a circuit which has special utility in the processing of Dual Tone Multiple Frequency signals in conjunction with telecommunications practices of the type found in the U.S.
2. Description of the Prior Art
A tone receiver is an electronic device used in telecommunications systems to convert a Dual Tone Multiple Frequency (DTMF) signal, into a multiple digit binary code. The format of a DTMF signal consists of high and low signal bands (697-941 Hz., and 1209-1633 Hz.). Each band carries a selected one tone of a predetermined set of four tones unique to the respective band. That is, there is simultaneous presence of two tones, with one in each band, and with each such one in each band being a selected one of four possible tones for the band. The DTMF signal system is sometimes referred to as the "2-of-8" signal system.
Generally, under U.S. telecommunication practices a DTMF signal burst must be rejected if it is less than 20 milliseconds in duration, and must be detected if it is 40 milliseconds or longer in duration. A common practice is to design tone receivers to detect signal bursts which are 30 milliseconds in duration or longer. The component of the tone receiver which does this is called a "coincidence timer". Its functions to time the duration of concurrent presence of the one signal in each channel of a valid DTMF signal. The importance in minimizing the delay in initiating the coincidence timer following reception of a tones burst is apparent. Stated another way, it is important that as much of a signal burst as possible be available for timing by the coincidence timer.
It is important for a change in DTMF signal to be recognized in the situation of an occurence without interruption of a shift from one valid DTMF signal to another valid DTMF signal. Also, because of the tendency of the selective filters, which are components of the tone detectors, to "ring", another very similar situation must be addressed. That is, the leading edge of a true DTMF signal must be recognized under the circumstances of a false signal which is due to such ringing followed immediately by a true DTMF signal.
Not all of the tones combinations of DTMF signals are used in any given tone receiver application. In fact, normal station-to-station call procedures exclude certain subsets of the tone combinations of the DTMF signal system. For example, one subset is used primarily in the communication of digital data rather than dialing, and another subset is used primarily in a private telephone line system of the U.S. Government. When the subsets are not applicable to the particular installation of the tone receiver it is desirable to inhibit their operation at the output of a tone receiver.
The multiple digit binary code output of a tone receiver is generally subjected to further processing in a utilization circuit. In many applications it is desirable for the tone receiver to provide an auxiliary output signal which contains signal intelligence to aid in the processing of the binary code which takes place in the utilization circuit.
Of course there is also a continuing desire to reduce the cost of such circuits, as by the use of forms of construction requiring a minimal number of components.
Accordingly, among the objects of the invention are the provision of:
1. A circuit for use in a tone receiver for converting the mutually exclusive digital outputs of a tone detector device into a desired binary code format and for providing output coupling, and which incorporates a large combination of features desirable for DTMF signal systems under telecommunications practices of the type found in the U.S.
2. A circuit for use in a tone receiver for converting the mutually exclusive outputs of a tone detector device into a desired binary code format and for providing output coupling, and which further incorporates therein the coincidence timer function.
3. A circuit in accordance with the preceding objective which operates in a way causing minimal delay in initiating the coincidence timer function.
4. A circuit for use in a tone receiver for converting the mutually exclusive outputs of a tone detector device into a desired binary code output format, and which provides an auxiliary tone receiver output signal that reliably indicates a change in input signal has occurred under the circumstances of the occurrence without interruption of shift from one valid input signal to another.
5. A circuit for use in a tone receiver for converting the mutually exclusive outputs of a tone detector device into a binary code output format, and which provides an auxiliary tone receiver output signal that reliably indicates the presence of the leading edge of a valid signal burst under the circumstances of a false signal due to "ringing" of a tone detector selective filter followed immediately by a true DTMF signal.
6. A circuit for use in a DTMF tone receiver for converting the mutually exclusive outputs of a tone detector device into a desired binary code, and which is readily modifiable after manufacture to accomodate different modes of the use of various subsets of the valid tone combinations of the DTMF signal system.
7. A circuit for use in DTMF tone receivers for converting the mutually exclusive outputs of a tone detector device into a desired binary code output format and for providing output coupling, and which further provides an auxiliary tone receiver output signal which contains signal intelligence to aid in the processing of the binary code output, which takes place in the utilization circuit.
8. A circuit for use in a tone receiver for converting the mutually exclusive digital outputs of a tone detector device into a desired binary code output format, and which is of a basic construction allowing the use of digital logic circuits employing minimal numbers of circuit components.